Germination
Germination is the process where growth emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However, the growth of a sporeling from a spore, for example the growth of hyphae from fungal spores, is also germination. In a more general sense, germination can imply anything expanding into greater being from a small existence or germ.
Seed germination
.jpg)
Germination is the first stage of the making of the seedling. The seed of a higher plant is a small package produced in a flower or cone containing an embryo and stored food reserves. Under favorable conditions, the seed begins to germinate, and the embryonic tissues resume growth, developing towards a seedling.
Dicot germination
The part of the plant that emerges from the seed first is the embryonic root, termed radicle or primary root. This allows the seedling to become anchored in the ground and start absorbing water. After the root, the embryonic shoot emerges from the seed. The shoot consists of three main parts: the cotyledons (seed leaves), the section of shoot below the cotyledons (hypocotyl), and the section of shoot above the cotyledons (epicotyl). The way the shoot emerges differs between plant groups.
Epigeous
In epigeous (or epigeal) germination, the hypocotyl elongates and forms a hook, pulling rather than pushing the cotyledons and apical meristem through the soil. Once it reaches the surface, it straightens and pulls the cotyledons and shoot tip of the growing seedlings into the air. Beans for example germinate this way.
Hypogeous
Another way of germination is hypogeous (or hypogeal) where the epicotyl elongates and forms the hook. In this type of germination, the cotyledons stay underground where they eventually decompose. Peas for example germinate this way.
Monocot germination
In monocot seeds, the embryo's radicle and cotyledon are covered by a coleorhiza and coleoptile, respectively. The coleorhiza is the first part to grow out of the seed, followed by the radicle. The coleoptile is then pushed up through the ground until it reaches the surface. There, it stops elongating and the first leaves emerge through an opening at its tip. Commonly, the primary root dies off and the plant develops shoot-borne roots.
Requirements for seed germination
Seed germination depends on many factors, both internal and external. The most important external factors include: water, oxygen, temperature, and the correct soil conditions. Every variety of seed requires a different set of variables for successful germination. This depends greatly on the individual seed variety and is closely linked to the ecological conditions in the plants' natural habitat.
Water
Germination requires moist conditions. Mature seeds are typically extremely dry and need to take up significant amounts of water before metabolism can resume. The uptake of water into seeds is called imbibition and leads to a marked swelling. The pressure caused by imbibing water aids in cracking the seed coat for germination. When seeds are formed, most plants store large amounts of food, such as starch, proteins, or oils, for the embryo inside the seed. When the seed imbibes water, hydrolytic enzymes are activated that break down these stored food resourc es and allow the seedling to germinate and grow non-photosynthetically until it reaches the light. Once the seedling starts growing, it requires a continuous supply of water and nutrients.
Oxygen
Most seeds respond best when water levels are enough to moisten the seeds but not soak them, and when oxygen is readily available. Once the seed coat is cracked, the germinating seedling requires oxygen for its metabolism. If the soil is waterlogged, it might cut off the necessary oxygen supply and prevent the seed from germinating as it prevents aerobic respiration, which is the main source for the seedling's energy until it starts to photosynthesize.
Temperature and light
Seeds germinate over a wide range of temperatures, with many preferring temperatures slightly higher than room-temperature. Often, seeds have a set temperature range for germination and will not germinate above or below a certain temperature. In addition, some seeds may require exposure to light or to cold temperature (vernalization) to break dormancy before they can germinate. As long as the seed is in its dormant state, it will not germinate even if conditions are favorable. For example, seeds requiring the cold of winter are inhibited from germinating if they never experience frost. Some seeds will only germinate when temperatures reach hundreds of degrees, as during a forest fire. Without fire, they are unable to crack their seed coats. Many seeds in forest settings will not germinate until an opening in the canopy allows then to receive sufficient light for the growing seedling.
Stratification
- Main article: Stratification (botany)
Seeds must be mature and environmental factors must be favorable before germination can take place. When a mature seed is placed under favorable conditions and fails to germinate, it is said to be dormant. Some seeds will not germinate (begin to grow) until they have been dormant for a while. The length of time plant seeds remain dormant can be reduced or eliminated by a simple seed treatment called stratification. Seeds should be planted promptly after stratification.
Stratification mimics natural processes that weaken the seed coat before germination. In nature, some seeds require particular conditions to germinate, such as the heat of a fire (e.g., many Australian native plants), or soaking in a body of water for a long period of time. Others have to be passed through an animal's digestive tract to weaken the seed coat and enable germination.
Hormonal control
Besides environmental factors, germination and dormancy in seeds are also influenced by plant hormones. The hormone absciscic acid affects seed dormancy and prevents germination, while the hormone gibberellin breaks dormancy and induces seed germination. This effect is used in brewing where barley is treated with gibberellin to ensure uniform seed germination to produce barley malt.
Seedling establishment
In some definitions, the appearance of the radicle marks the end of germination and the beginning of "establishment", a period that ends when the seedling has exhausted the food reserves stored in the seed. Germination and establishment as an independent organism are critical phases in the life of a plant when they are the most vulnerable to injury, disease, and water stress. The germination index can be used as an indicator of phytotoxicity in soils. The mortality between dispersal of seeds and completion of establishment can be so high, that many species survive only by producing huge numbers of seeds.
Pollen germination
Another germination event during the life cycle of gymnosperms and flowering plants is the germination of a pollen grain after pollination. Like seeds, pollen grains are severly dehydrated before being released to facilitate their dispersal from one plant to another. They consist of a protective coat containing several cells (up to 8 in gymnosperms, 2-3 in flowering plants). One of these cells is a tube cell. Once the pollen grain lands on the stigma of a receptive flower (or a female cone in gymnosperms), it takes up water and germinates. Pollen germination is facilitated by hydration on the stigma, as well as the structure and physiology of the stigma and style. Pollen can also be induced to germinate in vitro (in a petri dish or test tube).
During germination, the tube cell elongates into a pollen tube. In the flower, the pollen tube then grows towards the ovule where it discharges the sperm produced in the pollen grain for fertilization. The germinated pollen grain with its two sperm cells is the mature male microgametophyte of these plants.
Self-incompatibility
Since most plants carry both male and female reproductive organs in their flowers, there is a high risk for self-pollination and thus inbreeding. Some plants use the control of pollen germination as a way to prevent this selfing. Germination and growth of the pollen tube involve molecular signaling between stigma and pollen. In self-incompatibility in plants, the stigma of certain plants can molecularly recognize pollen from the same plant and prevents it from germinating.
Spore germination
Germination can also refer to the emergence of cells from resting spores and the growth of sporeling hyphae or thalli from spores in fungi, algae, and some plants.
Resting spores
In resting spores, germination involves cracking the thick cell wall of the dormant spore. For example, in zygomycetes the thick-walled zygosporangium cracks open and the zygospore inside gives rise to the emerging sporangiophore. In slime molds, germination refers to the emergence of amoeboid cells from the hardened spore. After cracking the spore coat, further development involves cell division, but not necessarily the development of a multicellular organism (for example in the free-living amboebas of slime molds).
Zoospores
In motile zoospores, germination frequently means a lack of motility and changes in cell shape, which allow the organism to become sessile.
Ferns and mosses
In plants such as bryophytes, ferns, and a few others, spores germinate into independent gametophytes. In the bryophytes (e.g. mosses and liverworts), spores germinate into protonemata, similar to fungal hyphae, from which the gametophyte develops. In ferns, the gametophytes are small, heart-shaped prothalli that can often be found underneath a spore-shedding adult plant.
See also
Bibliography
- P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2
External links
- Sowing Seeds A survey of seed sowing techniques.
- Seed Germination: Theory and Practice, Norman C. Deno, 139 Lenor Dr., State College PA 16801, USA. An extensive study of the germination rates of a huge variety of seeds under different experimental conditions, including temperature variation and chemical environment.
- Tree seed germination instructions Two methods of germinating tree seeds
